Photosynthetic acclimation to rising atmospheric carbon dioxide concentration

With rising level of CO2 in the atmosphere plants are expected to be exposed to higher concentration of CO2. Since, CO2 is a substrate limiting photosynthesis particularly in C3 plants in the present atmosphere, the impact of elevated CO2 would depend mainly on how photosynthesis acclimates or adjusts to the long term elevated level of CO2. Photosynthetic acclimation is a change in photosynthetic efficiency of leaves due to long term exposure to elevated CO2. This change in photosynthetic efficiency could be a biochemical adjustment that may improve the overall performance of a plant in a high CO2 environment or it could be due to metabolic compulsions as a result of physiological dysfunction. Acclimation has generally become synonymous with the word response, if long term exposure to elevated CO2 decreases the photosynthesis rate (Pn) at a given CO2 level, it is called negative acclimation, if it stimulates Pn at a given CO2 level, it is called positive acclimation. Photosynthetic acclimation is clearly revealed by comparing Pn of ambient and elevated CO2 grown plants at same level of CO2. Species level differences in acclimation to elevated CO2 have been reported. The physiological basis of differential photosynthetic acclimation to elevated CO2 is discussed in relation to the regulation of photosynthesis and photosynthetic carbon partitioning at cellular level.     

Ethylene insensitivity results in down-regulation of rubisco expression and photosynthetic capacity in tobacco

Little is known about the effect of hormones on the photosynthetic process. Therefore, we studied Rubisco content and expression along with gas exchange parameters in transgenic tobacco (Nicotiana tabacum) plants that are not able to sense ethylene. We also tested for a possible interaction between ethylene insensitivity, abscisic acid (ABA), and sugar feedback on photosynthesis. We measured Rubisco content in seedlings grown in agar with or without added sugar and fluridone, and Rubisco expression in hydroponically grown vegetative plants grown at low and high CO(2). Furthermore, we analyzed gas exchange and the photosynthetic machinery of transformants and wild-type plants grown under standard conditions. In the presence of exogenous glucose (Glc), agar-grown seedlings of the ethylene-insensitive genotype had lower amounts of Rubisco per unit leaf area than the wild type. No differences in Rubisco content were found between ethylene-insensitive and wild-type seedlings treated with fluridone, suggesting that inhibition of ABA production nullified the effect of Glc application. When larger, vegetative plants were grown at different atmospheric CO(2) concentrations, a negative correlation was found between Glc concentration in the leaves and Rubisco gene expression, with stronger repression by high Glc concentrations in ethylene-insensitive plants. Ethylene insensitivity resulted in plants with comparable fractions of nitrogen invested in light harvesting, but lower amounts in electron transport and Rubisco. Consequently, photosynthetic capacity of the insensitive genotype was clearly lower compared with the wild type. We conclude that the inability to perceive ethylene results in increased sensitivity to Glc, which may be mediated by a higher ABA concentration. This increased sensitivity to endogenous Glc has negative consequences for Rubisco content and photosynthetic capacity of these plants.     

彩叶植物叶片呈色分子机制研究进展

彩叶植物叶片呈现不同的颜色主要是受遗传因素和外部环境的共同作用,揭示彩叶植物叶片呈色机制对选育彩叶植物新品种和彩叶植物的应用推广具有重要理论和实践意义。目前对彩叶植物呈色机制的研究主要集中于叶片中色素变化、光合特性、叶片结构和环境条件等方面。该文主要対近年来有关彩叶植物叶片中叶绿素代谢途径、类胡萝卜素代谢途径、次生代谢途径、光合作用和叶绿体发育相关结构基因和转录因子调控机制的研究进展进行综述,并对以后的研究方向进行了展望,为培育彩叶植物新品种提供了理论基础,也为人工调控叶色以及叶色的定向遗传改良提供了参考。     

Enhanced photosynthesis and redox energy production contribute to salinity tolerance in Dunaliella as revealed by homology-based proteomics

Salinity is a major limiting factor for the proliferation of plants and inhibits central metabolic activities such as photosynthesis. The halotolerant green alga Dunaliella can adapt to hypersaline environments and is considered a model photosynthetic organism for salinity tolerance. To clarify the molecular basis for salinity tolerance, a proteomic approach has been applied for identification of salt-induced proteins in Dunaliella. Seventy-six salt-induced proteins were selected from two-dimensional gel separations of different subcellular fractions and analyzed by mass spectrometry (MS). Application of nanoelectrospray mass spectrometry, combined with sequence-similarity database-searching algorithms, MS BLAST and MultiTag, enabled identification of 80% of the salt-induced proteins. Salinity stress up-regulated key enzymes in the Calvin cycle, starch mobilization, and redox energy production; regulatory factors in protein biosynthesis and degradation; and a homolog of a bacterial Na(+)-redox transporters. The results indicate that Dunaliella responds to high salinity by enhancement of photosynthetic CO(2) assimilation and by diversion of carbon and energy resources for synthesis of glycerol, the osmotic element in Dunaliella. The ability of Dunaliella to enhance photosynthetic activity at high salinity is remarkable because, in most plants and cyanobacteria, salt stress inhibits photosynthesis. The results demonstrated the power of MS BLAST searches for the identification of proteins in organisms whose genomes are not known and paved the way for dissecting molecular mechanisms of salinity tolerance in algae and higher plants.     

Carbon metabolite feedback regulation of leaf photosynthesis and development

Photosynthesis is regulated as a two-way process. Light regulates the expression of genes for photosynthesis and the activity of the gene products (feedforward control). Rate of end-product use down-stream of the Calvin cycle, determined largely by nutrition and temperature, also affects photosynthetic activity and photosynthetic gene expression (feedback control). Whereas feedforward control ensures efficient light use, feedback mechanisms ensure that carbon flow is balanced through the pathways that produce and consume carbon, so that inorganic phosphate is recycled and nitrogen is distributed optimally to different processes to ensure growth and survival. Actual mechanisms are sketchy and complex, but carbon to nitrogen balance rather than carbon status per se is central to understanding carbon metabolite feedback control of photosynthesis. In addition to determining the activity of the metabolic machinery, carbon metabolite feedback mechanisms also regulate photosynthesis at the leaf level through the regulation of leaf development. This review summarizes the current sketchy, but growing, knowledge of the mechanisms through which carbon metabolite feedback mechanisms regulate leaf photosynthesis.     

臭氧胁迫对植物主要生理功能的影响

近年来,由于光化学反应的臭氧前体增加,全球植物受对流层臭氧(O3)胁迫的程度越来越严重。臭氧污染被认为是造成东欧、西欧和整个美国的大片森林衰退和枯死的主要原因。臭氧胁迫严重影响植物叶片对光能的利用,通过气孔限制和非气孔限制,导致其光合速率的降低,影响光合产物的产量。臭氧对植物的影响与植物体内代谢物质的积聚量紧密联系。臭氧胁迫引发植物的各种防御保护机制,刺激抗氧化系统,影响膜系统,改变其体内碳和矿质养分的吸收并引起它们的重新分配,诱导其基因表达的深层变化。为了适应臭氧胁迫环境,植物通过生理生化机制的调节来保证其生命活动。如细胞通过调节渗透物质的含量来保持渗透势的平衡;细胞内各种抗氧化酶活性增加,以清除自由基,避免或者减轻细胞受到伤害;改变代谢途径以保持能量储备和降低代谢速率。可见,生态环境对生物进化具有重要影响。这个观点将在臭氧胁迫对植物生理的影响中得到证实,也是生物进化论的另一种证据。综述了臭氧对光合生理、呼吸代谢、抗氧化系统、膜系统、矿质养分的吸收和分配与分子生理等主要生理功能的影响,并提出臭氧胁迫对植物生理影响的今后研究方向与未来研究热点是:(1)加强在植物个体和群落水平上臭氧胁迫对植物生理影响的研究;(2)臭氧影响下植物的基因调控和相关信号传递网络系统的机理;(3)通过分子标记、基因图谱、基因组学和转基因技术等方法研究选育适应臭氧胁迫环境的植物;(4)尽可能在接近自然条件的环境中开展研究;(5)臭氧胁迫对亚热带和热带森林及其树种主要生理功能影响的研究;(6)建立模型评估臭氧对植物的影响。     

高等植物铁氧还蛋白-NADP＋氧化还原酶研究进展

高等植物叶绿体定位的铁氧还蛋白-NADP＋氧化还原酶（LFNR）负责催化光合线性电子传递的最后一步反应,催化电子由还原态的铁氧还蛋白（Fd）传递给NADP＋。LFNR分布在叶绿体的3个不同的组分中,即叶绿体基质中、类囊体膜上和叶绿体内膜上。最近的研究表明,大多数膜定位的LFNR并非光合作用所必需的,叶绿体基质中的LFNR足以维持光合作用的正常进行。叶绿体中的两个蛋白——Tic62和TROL作为LFNR的锚定蛋白,可以与LFNR在类囊体膜上形成高分子量的蛋白复合体。Tic62-LFNR复合体主要负责在夜间保护LFNR的活性,但它不直接在光合作用中起作用。然而,TROL-LFNR复合体对植物的光合作用有一定的影响。本文将概述植物LFNR的最新研究进展。     

Sugar repression of photosynthesis: the role of carbohydrates in signalling nitrogen deficiency through source:sink imbalance

The aim of this work was to examine whether carbohydrates are involved in signalling N deficiency through source:sink imbalance. Photosynthetic metabolism in tobacco was studied over 8 d during the withdrawal of N from previously N-sufficient plants in which the source:sink ratio was manipulated by shading leaves on some of the plants. In N-sufficient plants over this time-scale, there was a small decline in photosynthetic rate, Rubisco protein and amino acid content, with a larger decrease in carbohydrate content. Withdrawal of N from the growing medium induced a large decrease in the rate of photosynthesis (35% reduction after 8 d under the growing conditions, with a reduction also apparent at high and low measuring CO₂), which was caused by a large decrease in the amount of Rubisco protein (62% after 8 d) and Rubisco activity. Higher amounts; of hexoses preceded the loss of photosynthetic activity and sucrose and starch accumulation. Reduction of the sourcersink ratio by shading prevented the loss of photosynthetic activity and the increase in hexoses and other carbohydrates. These data indicate that the reduction of photosynthesis that accompanies N deficiency in intact plants has the characteristics of sugar repression of photosynthesis observed in model systems, but that the accumulation of hexose prior to the decline in photosynthesis is small. The possibility that sugar repression of photosynthesis under physiological conditions depends more crucially on the C:N status of leaves than the carbohydrate status alone is discussed.     

The coordinated role of ethylene and glucose in sulfur-mediated protection of photosynthetic inhibition by cadmium

Ethylene controls photosynthesis and induces tolerance of plants to metal stress. However, little is known about the interaction between ethylene, photosynthesis and sulfur (S) availability under cadmium (Cd) stress. Recently, we reported that ethylene controls photosynthesis by increasing glutathione (GSH) synthesis with sufficient-S availability under Cd stress. Plants treated with Cd were less sensitive to ethylene and showed photosynthetic inhibition. Ethylene sensitivity of plants was increased with exogenously-sourced ethylene or with sufficient-S application resulting in induced GSH synthesis and alleviation of photosynthetic inhibition by Cd. In this addendum we present some additional data indicating that ethylene regulates photosynthesis by reducing glucose (Glc) sensitivity, thus reducing the Glc-mediated photosynthetic repression.     

Regulation of photosynthesis through source: sink imbalance in citrus is mediated by carbohydrate content in leaves

In citrus, the occurrence of a sink effect on photosynthesis (A) is controversial. Leaf carbohydrates and photosynthetic rates in field-grown trees of Satsuma mandarin ( Citrus unshiu [Mak.] Marc.) cv. Okitsu, were measured to elucidate whether or not the demand for photoassimilates regulates A. The data indicated that the source-sink imbalances induced by different treatments altered both soluble (sucrose, glucose and fructose) and insoluble carbohydrates in leaves, as well as photosynthetic rates. In general, girdling and defruiting increased starch and reduced photosynthesis, whereas source-limiting conditions imposed through partial defoliations had the opposite effect. These results are compatible with the assumption that a lack of sink activity leads to carbohydrate accumulation and feedback inhibition of A, and vice versa. Further evidence supporting a source-sink effect on A was provided by measurements of the dry matter:leaf area ratio, since defoliations, for example, increased this ratio. The in vivo sucrose supplementation to plants with different source:sink ratios (control, defoliated, girdled and defruited plants) increased carbohydrates and reduced photosynthesis. This suggests that sugars may have, per se, the potential to repress photosynthetic rates in intact plants with active sinks. Based on these results we propose that sugar accumulation in citrus leaves causes a feedback inhibition of A.     

Chloroplast redox signals: how photosynthesis controls its own genes

The photosynthetic apparatus of higher plants and algae is composed of plastid- and nuclear-encoded components, therefore the expression of photosynthesis genes needs to be highly coordinated. Expression is regulated by various factors, one of the most important of which is light. Photosynthesis functions as a sensor for such light signals, and the redox state of photosynthetic electron transport components and redox-active soluble molecules act as regulating parameters. This provides a feedback response loop in which the expression of photosynthesis genes is coupled to the function of the photosynthetic process, and highlights the dual role of photosynthesis in energy fixation and the reception of environmental information.     

Molecular evolution of rbcL in the mycoheterotrophic coralroot orchids (Corallorhiza Gagnebin, Orchidaceae)

The RuBisCO large subunit gene (rbcL) has been the focus of numerous plant phylogenetic studies and studies on molecular evolution in parasitic plants. However, there has been a lack of investigation of photosynthesis gene molecular evolution in fully mycoheterotrophic plants. These plants invade pre-existing mutualistic associations between ectomycorrhizal trees and fungi, from which they obtain fixed carbon and nutrients. The mycoheterotrophic orchid Corallorhiza contains both green (photosynthetic) and non-green (putatively nonphotosynthetic) species. We sequenced rbcL from 31 accessions of eight species of Corallorhiza and hypothesized that some lineages would have pseudogenes resulting from relaxation of purifying selection on RuBisCO's carboxylase function. Phylogenetic analysis of rbcL+ITS gave high jackknife support for relationships among species. We found evidence of pseudogene formation in all lineages of the Corallorhiza striata complex and in some lineages of the C. maculata complex. Evidence includes: stop codons, frameshifts, decreased d(S)/d(N) ratios, replacements not observed in photosynthetic species, rate heterogeneity, and high likelihood of neutral evolution. The evolution of rbcL in Corallorhiza may serve as an exemplary system in which to study the effects of relaxed evolutionary constraints on photosynthesis genes for >400 documented fully mycoheterotrophic plant species.     

Sugar sensing in higher plants.

Sugar repression of photosynthetic genes is likely a central control mechanism mediating energy homeostasis in a wide range of algae and higher plants. It overrides light activation and is coupled to developmental and environmental regulations. How sugar signals are sensed and transduced to the nucleus remains unclear. To elucidate sugar-sensing mechanisms, we monitored the effects of a variety of sugars, glucose analogs, and metabolic intermediates on photosynthetic fusion genes in a sensitive and versatile maize protoplast transient expression system. The results show that sugars that are the substrates of hexokinase (HK) cause repression at a low concentration (1 to 10 mM), indicating a low degree of specificity and the irrelevance of osmotic change. Studies with various glucose analogs suggest that glucose transport across the plasma membrane is necessary but not sufficient to trigger repression, whereas subsequent phosphorylation by HK may be required. The effectiveness of 2-deoxyglucose, a nonmetabolizable glucose analog, and the ineffectiveness of various metabolic intermediates in eliciting repression eliminate the involvement of glycolysis and other metabolic pathways. Replenishing intracellular phosphate and ATP diminished by hexoses does not overcome repression. Because mannoheptulose, a specific HK inhibitor, blocks the severe repression triggered by 2-deoxyglucose and yet the phosphorylated products per se do not act as repression signals, we propose that HK may have dual functions and may act as a key sensor and signal transmitter of sugar repression in higher plants.     

植物光合作用的三维特性研究进展

光合作用是地球上最重要的化学反应。虽然针对植物光合作用已经进行了广泛深入的研究, 但从三维层面探讨植物叶片光合功能及其调节作用的工作较少。叶片结构、光合机构组分、叶片内光能吸收和传递均具有明显的三维特性, 极大影响叶片内CO₂转运、叶肉细胞的电子传递和碳同化, 进而使叶片光合功能及其调控表现出复杂的三维特征。因此, 从三维角度分析叶片光合特性有助于理解光合作用机理, 也能够为提高植物光合作用效率提供理论支持。     

Arbuscular Mycorrhizal Symbiosis with Arundo donax Decreases Root Respiration and Increases Both Photosynthesis and Plant Biomass Accumulation

The effect of arbuscular mycorrhiza (AM) symbiosis on plant growth is associated with the balance between costs and benefits. A feedback regulation loop has been described in which the higher carbohydrate cost to plants for AM symbiosis is compensated by increases in their photosynthetic rates. Nevertheless, plant carbon balance depends both on photosynthetic carbon uptake and respiratory carbon consumption. The hypothesis behind this research was that the role of respiration in plant growth under AM symbiosis may be as important as that of photosynthesis. This hypothesis was tested in Arundo donax L. plantlets inoculated with Rhizophagus irregularis and Funneliformis mosseae. We tested the effects of AM inoculation on both photosynthetic capacity and in vivo leaf and root respiration. Additionally, analyses of the primary metabolism and ion content were performed in both leaves and roots. AM inoculation increased photosynthesis through increased CO₂ diffusion and electron transport in the chloroplast. Moreover, respiration decreased only in AM roots via the cytochrome oxidase pathway (COP) as measured by the oxygen isotope technique. This decline in the COP can be related to the reduced respiratory metabolism and substrates (sugars and tricarboxylic acid cycle intermediates) observed in roots.